bgerror(n) Tcl Built-In Commands bgerror(n)
NAME
bgerror - Command invoked to process background errors
SYNOPSIS
bgerror message
DESCRIPTION
Release 8.5 of Tcl supports the interp bgerror command, which allows
applications to register in an interpreter the command that will handle
background errors in that interpreter. In older releases of Tcl, this
level of control was not available, and applications could control the
handling of background errors only by creating a command with the par-
ticular command name bgerror in the global namespace of an interpreter.
The following documentation describes the interface requirements of the
bgerror command an application might define to retain compatibility
with pre-8.5 releases of Tcl. Applications intending to support only
Tcl releases 8.5 and later should simply make use of interp bgerror.
The bgerror command does not exist as built-in part of Tcl. Instead,
individual applications or users can define a bgerror command (e.g. as
a Tcl procedure) if they wish to handle background errors.
A background error is one that occurs in an event handler or some other
command that did not originate with the application. For example, if
an error occurs while executing a command specified with the after com-
mand, then it is a background error. For a non-background error, the
error can simply be returned up through nested Tcl command evaluations
until it reaches the top-level code in the application; then the appli-
cation can report the error in whatever way it wishes. When a back-
ground error occurs, the unwinding ends in the Tcl library and there is
no obvious way for Tcl to report the error.
When Tcl detects a background error, it saves information about the
error and invokes a handler command registered by interp bgerror later
as an idle event handler. The default handler command in turn calls
the bgerror command . Before invoking bgerror, Tcl restores the error-
Info and errorCode variables to their values at the time the error
occurred, then it invokes bgerror with the error message as its only
argument. Tcl assumes that the application has implemented the bgerror
command, and that the command will report the error in a way that makes
sense for the application. Tcl will ignore any result returned by the
bgerror command as long as no error is generated.
If another Tcl error occurs within the bgerror command (for example,
because no bgerror command has been defined) then Tcl reports the error
itself by writing a message to stderr.
If several background errors accumulate before bgerror is invoked to
process them, bgerror will be invoked once for each error, in the order
they occurred. However, if bgerror returns with a break exception,
then any remaining errors are skipped without calling bgerror.
If you are writing code that will be used by others as part of a pack-
age or other kind of library, consider avoiding bgerror. The reason
for this is that the application programmer may also want to define a
bgerror, or use other code that does and thus will have trouble inte-
grating your code.
EXAMPLE
This bgerror procedure appends errors to a file, with a timestamp.
proc bgerror {message} {
set timestamp [clock format [clock seconds]]
set fl [open mylog.txt {WRONLY CREAT APPEND}]
puts $fl "$timestamp: bgerror in $::argv '$message'"
close $fl }
SEE ALSO
after(n), errorCode(n), errorInfo(n), interp(n)
KEYWORDS
background error, reporting
Tcl 7.5 bgerror(n)
tcl 8.6.1 - Generated Mon Sep 30 07:36:23 CDT 2013